The present invention relates generally to the field of control of electroplating baths. In particular, the present invention relates to the control of electroplating baths using real-time monitoring of the bath components.
Electroplating baths for copper and other metals are typically aqueous, or mostly aqueous, solutions composed of metal compounds or salts, ionic electrolytes, and various additives such as brighteners, suppressors, levelers, accelerators, surfactants, defoamers, and the like. These electroplating baths, which are used to deposit metals or semimetals such as copper, nickel, gold, palladium, platinum, ruthenium, rhodium, tin, zinc, antimony, or alloys such as copper-tin (brass), copper-zinc (bronze), tin-lead, nickel-tungsten, cobalt-tungsten-phosphide, and the like are used in applications such as the fabrication of electronic devices and components, such as conductive circuits for printed circuit boards, multichip modules, semiconductor devices and the like.
Reliable operation of these electroplating baths in a manufacturing process requires that methods of analysis are employed to determine the appropriate concentrations of the reagent species for bath startup. These analysis methods are also used to determine the concentrations of species in the bath during operation, often with on-line feedback control, to allow the components of the bath to be monitored and adjusted as required to maintain concentrations within pre-determined limits Bath analysis methods are also used to determine the chemical identity and concentrations of species that are produced in the bath as a consequence of chemical and electrochemical reactions that take place during bath operation and/or idling. Such bath analysis methods include cyclic voltammetric stripping (xe2x80x9cCVSxe2x80x9d), cyclic pulse voltammetric stripping (xe2x80x9cCPVSxe2x80x9d), open circuit potential (xe2x80x9cOCPxe2x80x9d) measurement, AC impedance, high pressure liquid chromatography (xe2x80x9cHPLCxe2x80x9d), ion chromatography (xe2x80x9cICxe2x80x9d), titrimetry, gravimetric analysis, optical spectroscopy, and the like. See, for example, U.S. Pat. No. 5,223,118 (Sonnenberg et al.) and U.S. Pat. No. 4,917,774 (Fisher). Chromatographic techniques such as HPLC and IC are useful laboratory methods, for analyzing various components of plating baths, but they have not been widely implemented in commercial bath analysis systems. Titrimetric and gravimetric techniques are more widely used than chromatographic methods, but these methods require the use of various additional chemistries (titrants, complexants, precipitants) and are difficult to implement in an on-line, real-time configuration.
Electrochemical techniques such as CVS and CPVS have been most widely used in commercial applications for analysis of plating baths because these methods are reliable and are particularly well-suited to on-line, real-time analysis. However, the electrochemical techniques are also limited in several aspects. Each technique measures a current flow in response to a changing applied potential across an electrochemical cell containing the plating solution. The current is a response that reflects the aggregate of all of the electrochemical reactions that occur in the cell at a given potential. These techniques are generally unable to distinguish between different or competing reactions that occur at the same potential. These electrochemical techniques also are not specific to particular chemical species in the solution, so the changing concentration of individual species cannot be directly measured.
In the operation of commercial electroplating baths, it is very important to be able to measure and control the individual components of the plating bath, particularly the organic additives. These materials are typically present in the plating bath in small amounts relative to the metal salts or electrolytes. However, the additives play a major role in controlling both the characteristics of the deposition process such as the plating rate, as well as the physical properties of the deposit such as uniformity, grain size, ductility, stress, surface roughness, and the like. In a typical electroplating bath two, three, or even more additives may be deliberately formulated into the bath to provide the desired plating characteristics and deposit properties. Techniques such as CVS, CPVS, or OCP can only measure the overall electrochemical behavior of a plating bath but can not independently determine the concentrations of the various additive species in the bath without resorting to complex analysis schemes that involve the use of special calibration solutions or other similar approaches. Additionally, additives are often small organic molecules or polymers that either undergo electrochemical, chemical, or other reactions (such as surface adsorption) under the applied potential conditions at which the electroplating process takes place. These reactions can change some portion of the original additive species to different species. The relative proportions and chemical or electrochemical activities of these new species can change over time, depending on the conditions of use of the plating bath. The changing concentrations of species and their activities affects the electrochemical behavior of the plating bath and ultimately can affect the properties of the deposits produced from the bath. It is very difficult to determine the nature of these reaction products in the electroplating bath by conventional electrochemical methods or measure their changing concentrations over time, yet these species may actually be the most important ones to measure and control in order to optimize the properties of the electrodeposited films.
Differential electrochemical mass spectrometry (xe2x80x9cDEMSxe2x80x9d) has been used to detect various chemical species, such as the evolution of carbon dioxide gas from an electrochemical reaction. This technique has never been applied to the analysis of organic components in an electroplating bath.
Thus, there is a continuing need for a method that can more accurately determine the specific nature of the additive species and their reaction products that are present in an electroplating bath and to measure their concentrations on-line, in real-time, and over time as the electroplating reaction proceeds.
It has been surprisingly found that the present method provides on-line, real-time analysis of electroplating bath components. According to the present method, all organic additives in an electroplating bath may be monitored simultaneously.
In one aspect, the present invention provides a method for determining the level of components in an electroplating bath including the steps of: a) obtaining a plurality of solutions wherein each solution has known and different concentrations of an analyte, but where the quantity of the analyte in each solution differs from the quantity in the other solutions; b) providing an apparatus having a first chamber and a second chamber, the first chamber being separated from the second chamber by a liquid-impermeable, gas-permeable membrane; c) introducing each solution individually into the first chamber and carrying out a predetermined sequence of steps including: i) reducing the pressure in the second chamber relative to the first chamber to produce a gas stream; ii) directing at least a portion of the gas stream to a mass spectrometer; iii) measuring a characteristic mass/charge peak for the analyte; d) for each solution, correlating the quantity of analyte with the measurement of the characteristic mass/charge peak; e) introducing a bath having an unknown quantity of the analyte into the first chamber; f) performing the predetermined sequence of steps; and g) choosing from the correlation in step d) a quantity of the analyte which corresponds to the recorded characteristic mass/charge peak measurement for the analyte.
In a second aspect, the present invention includes an electroplating system including an electroplating tank containing an electroplating bath, the tank having an outlet for directing a portion of the electroplating bath to an apparatus for determining the level of components in the electroplating bath, the apparatus including a first chamber separated from a second chamber by a liquid-impermeable, gas-permeable membrane, a means for reducing the pressure in the second chamber relative to the first chamber to produce a gas stream, and a means for directing at least a portion of the gas stream to a mass spectrometer.
In a third aspect, the present invention provides a method for electrolytically depositing metal on a substrate including the steps of: a) contacting the substrate with an electroplating bath including a source of metal ions, and electrolyte and one or more organic additives; b) subjecting the electroplating bath to sufficient current density for a period of time sufficient to deposit a desired thickness of metal on the substrate; and c) monitoring the one or more organic additives by i) obtaining a plurality of solutions wherein each solution has known and different concentrations of an organic additive, but where the quantity of the organic additive in each solution differs from the quantity in the other solutions; ii) providing an apparatus having a first chamber and a second chamber, the first chamber being separated from the second chamber by a liquid-impermeable, gas-permeable membrane; iii) introducing each solution individually into the first chamber and carrying out a predetermined sequence of steps including: aa) reducing the pressure in the second chamber relative to the first chamber to produce a gas stream; bb) directing at least a portion of the gas stream to a mass spectrometer; cc) measuring a characteristic mass/charge peak for the organic additive; iv) for each solution, correlating the quantity of organic additive with the measurement of the characteristic mass/charge peak; v) introducing a portion of the electroplating bath having an unknown quantity of the organic additive into the first chamber; vi) performing the predetermined sequence of steps; and vii) choosing from the correlation in step iv) a quantity of the organic additive which corresponds to the recorded characteristic mass/charge peak measurement for the organic additive.